CICPs are colorants comprising a solid solution in which two or more oxides are combined. Since they have excellent weatherability, acid resistance, and heat resistance due to their extreme chemical and physical stability, the pigments are widely used in the applications that require durability such as paints, building materials, resin and the like.
CICP is composed of a combination of various oxides. Among them, chromium oxide provides various durability at higher levels, and a wide variety of color tones by combinations with other oxides. Therefore, various chromium-containing CICPs have been put to practical use, and used in a broad range of areas.
Chromium ion in a complex oxide is stable in the trivalent state. However, in recent years, there have been growing concerns about harmful effects on the environment and human health that a small amount of hexavalent chromium eluting from the chromium-containing CICP has, due to its toxic and carcinogenic properties. In EU member countries, for instance, the use of hazardous substances in electrical and electronic equipment has been restricted; any products having a hexavalent-chromium elution amount of over 1000 ppm cannot be put on the market
The elution amount of hexavalent chromium from the chromium-containing CICP greatly varies depending on the chromium content, the types of co-existing materials in the solid solution, the calcination condition, the water rinse condition and the like. For instance, the elution amount of hexavalent chromium from Pigment Black 28 sometimes exceeds 1000 ppm.
To reduce the elution amount of hexavalent chromium from the chromium-containing CICP, a method of sufficiently rinsing the pigment with water, treating the pigment with a reducing agent such as sodium sulfite, or insolubilization by converting it into zinc chromate is commonly used. However, with the above-mentioned methods, the manufacturing process of the pigment is cumbersome and the elution of hexavalent chromium from the pigment cannot be satisfactorily reduced.
JP Publication of unexamined application No. 8-27393 discloses a process for reducing an elution amount of hexavalent chromium by bringing a pigment into contact with hydrated silica and a reducing substance in a slurry of chromium-containing CICP to treat the pigment with these substances. However, the process is cumbersome, and therefore not suited for manufacturing general-purpose products.
U.S. Pat. No. 2,309,173 discloses a black pigment of the copper-chromium type, obtained by adding 15% or less of an oxide of iron, nickel, cobalt, manganese, molybdenum, tungsten, vanadium or uranium to a mixture of copper oxide and chromium oxide, and then calcining the resulting mixture at a temperature of 800 to 1100° C. However, the black pigment that does not contain much manganese therein does not exhibit excellent blackness and tinting strength.